DESCRIPTION OF THE PRIOR ART
Repairable systems for providing strain relief to flexible cables at the point of transition to hardware have fallen into two categories: mechanical clamps and shrinkable boots. Each of these systems has its distinct advantages and drawbacks, but neither provides a complete solution for repairable strain relief and sealing.
Saddle clamps are the most common method of providing strain relief. Such clamps typically consist of two parallel bars joined by screws passing between them at each end. By turning the screws, the bars move closer together, thereby providing a clamping force. The bars are held to end fitting hardware by clamping to an extended portion of the end fitting hardware that is placed between the two bars. This method of clamping is advantageous for two reasons. First, it can be installed and prepared using only a screwdriver. Second, when the clamps are open, the end fitting hardware can be slid down the outside of the cable, allowing the end of the cable to be accessed and repaired, such as is typical with electrical backshell hardware.
However, saddle clamps also have many disadvantages, including:
The clamping ability of saddle clamps is marginal. They clamp only on two sides, forcing round cables into an oval configuration.
They have a narrowly limited size range. When cables are too small, the clamps tighten on the end fitting tabs before they are tight on the cable--leaving the cable loose. When the cable is too big, the clamps tighten on the cable before getting tight on the hardware tabs, leaving the hardware loose. The only way to insure solid clamping is to oversize the clamp, and use a rubbery grommet or tape around the wire bundle.
Including the grommet, the screws and two bars, the saddle clamp has five separate components, excluding the hardware body and washers.
Saddle clamps do not provide a moisture seal between an end fitting and the cable jacketing, and additional components must be added to achieve good sealing. Saddle clamps also do not provide an EMI seal between the end fitting and a cable bulk shielding, and additional components must be added for this purpose. Good grounding is achieved only with the addition of a jumper cable attched between a cable shield and the clamp screws.
Without special thread inserts, or lockwashers, saddle clamps will come loose under vibration.
Saddle clamps have sharp screw ends protruding so as to be a potential source of cuts and scrapes to any wires, surface finishes, or mechanic's hands in their vicinity. Their bulk profiles with protruding points are aesthetically unpleasant and give an unfinished appearance.
Despite all of these negative features, saddle clamps remain popular because they are easy to repair, and do not require any special tooling.
Shrinkable boots are one alternative to saddle clamps. They have a sleek low profile with no protruding surfaces. When used lined with adhesives they can provide moisture sealing along with some axial strain relief. Shrinkable boots can accommodate a wide range of sizes. Shrinkable boots address many of the aesthetic problems of saddle clamps, but lack their ease of repair. Once installed, a boot cannot be repaired, and must be cut off and replaced with a new boot. Heat shrinkable boots require the use of a special heat gun for proper installation or repair. In many cases heat guns cannot be used around fueled aircraft, resulting in additional installation and repair problems. Although several shrinkable systems have been proposed that can provide EMI shielding, they have generally not found wide acceptance because of poor shielding and grounding. The recovery force of shrinkable systems is insufficient to provide solid electrical grounding.
A third type of repairable strain relief system is the wedge and grommet type. These systems utilize a rubber grommet or bushing that is compressed radially inward by a compression nut. A circular bushing is slid over a cable and sandwiched between the end fitting body and the threaded compression nut. The nut slides over the grommet and screws onto the body, so that a wedged surface on the nut can drive the grommet down around the cable jacketing. While these systems offer repairable sealing and strain relief, their size range is very limited so that the grommet needs to be carefully sized to fit the cable. Nut and wedge systems can be expanded to include EMI shielding, however, this increases the number of components, and the complexity of the assembly, requiring the installer to handle multiple components.
One method of clamping circular items that has been in use in the hydraulic industry is the worm drive clamp, commonly known as a hose clamp. Hose clamps are popular because they are extremely easy to operate, requiring only a screwdriver. They are reusable and can accommodate a wide range of hose sizes. They provide a strong radial clamping force capable of sealing to high pressures. The clamping performance of hose clamps has been established from years of use, and they are commonly used in jet aircraft engines and in areas of high vibration and temperature extremes. They are widely available and relatively low in cost.
Hose clamps as they currently exist are not widely used in the electrical interconnect industry as a means for providing sealing or strain relief. There are several reasons for this, some functional, others pertaining to industry preferences. To secure a cable to an end fitting, a circular ferrule portion of the end fitting must first be placed under the cable jacket, and then the hose clamp can be used to compress the jacketing against the ferrule. While the thin cable jacketing is secured and sealed, the internal wires remain unsupported from pulling or bending. Further, the diameter of the ferrule must be closely sized to the diameter of the cable jacketing, limiting the range of sizes it can accommodate. It is conceivable that a hose clamp could be used to strap a cable bundle to a post or arm extending from an end fitting, however this would not allow for sealing or EMI shielding. Using the clamp as a strap is also undesirable because a loose end of the band extends past the housing when the clamp is tightened. This loose end can catch on wires, cause scrapes in neighboring components, and generally gives an unfinished appearance to a cable assembly. In the electrical interconnect industry hose clamps are regarded as bulky items that give a "hacked" together appearance. To some extent this reputation is undeserved, and the prejudice limits their use when they might otherwise be appropriate.
There is no single product that can provide both strain relief and sealing in a system that is easily installed and repaired, in a wide range of cable sizes, and requiring no special tools. Further, no single product exists that can provide strain relief and EMI shielding in a system that is easily installed and repaired, in a wide range of cable sizes, and requiring no special tools.